In building structures, it is often aesthetically desirable to cover large portions of the outside of the structures with as much glass and as little outside framing elements as possible, thereby providing the structures with a smooth and unbroken outside surface appearance. Conventionally, a structural adhesive bond is provided between the building structure and the inside surfaces of glass panels to attach the panels to the building structure, thus reducing or eliminating the need for permanent outside retention members. Such bonding configurations are commonly known as “Structural Silicone Glazing” or “SSG” systems.
Typical SSG systems fall into two major classes: two-sided and four-sided. Four-sided SSG systems typically include a plurality of vertical structural mullions in combination with a plurality of horizontal structural mullions, which combine to form a mullion framework having a plurality of panel-shaped openings which are slightly smaller than the glass panels to be supported. The mullion framework is fixed about the exterior of a building structure. Each glass panel is positioned adjacent to the exterior surface of the mullion framework and over a corresponding panel-shaped opening by a plurality of retaining clips, such that the edges of the panel slightly overlap the panel-shaped opening and a small gap exists between the inside surfaces of the glass panel and the framework. Structural adhesive, typically structural silicone, is then applied into the gap. After the silicone adhesive cures, it provides a structural bond between the mullion framework and the glass panel which can support the glass panel for lateral loads. For weatherproofing purposes, additional silicone adhesive is then applied from the outside of the building into gaps created by the abutting edges of the adjacent glass panels.
Two-sided SSG systems differ in that a structural adhesive bond as described above is provided along two (usually vertical) opposing edges of the glass panels. In two-sided SSG systems, the two edges not being structurally bonded to the mullion framework are retained by other means, typically mechanically fastened, and thus support the dead load of the panels. This is normally done by conventional window glazing means which enclose the entire edge of the glass panels, thus not providing the smooth continuous appearance of four-sided SSG systems.